1. Field of the Invention
This invention relates to the drying and burning of wet waste materials such as sewage sludge.
2. State Of The Art
Conventional wastewater treatment processes remove pollutants from the wastewater and generate slurries of the removed pollutants. These slurries are generally known as sludge, which contains organic and inorganic solids suspended in a high percentage of water. Disposal of the sludge is accomplished by known methods, and one such method includes dewatering the sludge by filtration or centrifugation and incineration of the dewatered sludge. It is generally recognized that filtration of sludge removes only a fraction of the water, and substantial quantities of water remain with the sludge even after filtration. Therefore, it is desirable to remove additional water from the sludge prior to incineration so that it is not necessary to heat substantial quantities of water in the incineration step. Other materials such as municipal refuse having a substantial percentage of organic materials, can also be dried and incinerated in combination with sludge.
One conventional system for drying and incinerating wet sewage sludge is a multiple hearth furnace. A conventional multiple hearth furnace includes a substantially vertically disposed cylindrical vessel having an inlet at its upper end and an outlet at its lower end. A plurality of hearths are located within the vessel perpendicular to its axis, and spaced apart from one another. A center column is disposed to rotate in the center of the furnace parallel to the axis of the furnace, and a plurality of rabble arms are afixed to the center column parallel to the hearths. When the center column rotates the rabble arms urge material to travel across the hearths, and the hearths are constructed so that material introduced at the upper end of the furnace travels from hearth to hearth downwardly through the furnace in a serpentine path.
In operation of a conventional multiple hearth furnace, wet sludge is introduced into the upper part of the furnace and moved downwardly. Burners affixed to the furnace burn fuel such as natural gas to heat the sludge to dry it. In the lower part of the furnace fuel is burned to burn the sludge thereby forming hot gases which rise to aid in drying the sludge in the upper hearth spaces. It should be understood that the center column and the rabble arms are hollow so that ambient air introduced into the lower end of the center column flows upwardly therethrough and acquires heat from the furnace. The heated air leaves the upper part of the center column and is conveyed therefrom back into the furnace to provide combustion air.